1. Field of the Invention
This invention relates to a capacitance bridge system and more particularly to the use of a capacitive bridge and related cicuitry for measuring the void content of a solid dielectric.
2. Description of the Prior Art
High voltage electrical insulation, as used for example, in motors, generators, or cast resin bushings, usually contain some voids. These voids may arise for instance, as a result of imperfect filling of spaces between mica flakes or splitting by the impregnating resin, or from bubbles in cast resins. When the solid insulation is later subjected to high voltage alternating electrical stress, gas discharges, commonly called partial discharges or corona, will occur in the residual gas in these voids, eroding the surrounding resins and ultimately causing insulation failure. For this reason, measurement of void content is a well recognized tool for evaluating insulation quality.
The measurement of void content can be effected electrically by making the insulation the dielectric of a capacitor whose capacitance is measured at low voltages, insufficient to cause void ionization, and also at high voltages where the voids are ionized and act in effect like short circuits. The measurement of the low voltage capacitance and the high voltage capacitance provides a means for estimating the total void content. In an article entitled "A Capacitance Bridge Method For Measuring Integrated Corona-Charge Transfer and Power Loss Per Cycle" by T. W. Dakin and P. J. Malinaric, Paper 60-97, AIEE, 1960 Winter General Meeting, a method which utilizes a capacitance bridge and an oscillograph for estimating total void content is described in detail.
The most satisfactory method for measuring low voltage capacitance and the difference between the low voltage capacitances and the high voltage capacitances is a capacitance bridge. In operation, this capacitance bridge is balanced at a voltage low enough so that no void discharge occurs; and the output voltage is displayed as the vertical deflection on an oscillograph. The horizontal deflection on the oscillograph is proportional to the applied sinusoidal high voltage wave. When balanced at low voltage, the display will be a horizontal straight line. At applied voltages considerably higher than the point at which initial void ionization occurs, a parallelogram-shaped figure will be displayed on the oscillograph. The slope of the sides of this figure is proportional to the difference between the high voltage capacitance and the low voltage capacitance of the dielectric sample. In most practical insulation samples, many voids of different sizes and hence of different ionization voltages will be present, causing some curvature of the sides of the parallelogram at low voltages. That is, the sides of the parallelogram near the obtuse angles will be curved. In such case, measurements must be made at voltages high enough to give a relatively straight portion of the parallelogram sides, near voltage crests, to accurately portray the slope of this portion which is used for calculating the void fraction. It has been prior art practice to photograph the parallelogram display for each voltage of interest, and later make a measurement on the photograph to determine the slope of the parallelogram sides. The slope yields the difference between the high voltage capacitance and the low voltage capacitance from which the void content can be calculated. This prior art procedure has the disadvantages of costing time and film for the actual photograph and requiring time for later measurements on the photograph. In addition to these disadvantages, there is an undesirable time delay imposed and there is also the possibility of misidentifying, mislaying or misinterpreting the photographs showing the parallelogram.